Displays are used on notebook PCs, televisions, and other electronic devices. Like most electronic devices, displays must be calibrated to accurately display video and graphic images. For example, the common voltage of a display is calibrated for optimum viewing and operation. Without proper calibration, the image on the display can substantially flicker. In some types of displays, such as liquid crystal displays (LCDs), e-ink displays, and electro-wetting displays, the pixel material can be damaged if the common voltage is not set correctly.
Some displays are characterized by a common voltage (VCOM), herein referred to as VCOM displays. The VCOM voltage is applied to the backplane of a VCOM display panel. Application of the VCOM voltage allows for adjustment of the absolute voltage applied to the pixel, when the pixel is turned on. When the VCOM voltage is properly adjusted, approximately the same absolute voltage is applied to the pixel during the entire cycle of the analog source voltage that is supplied to the pixel. Providing a substantially constant absolute on-voltage to the pixel substantially eliminates perceived flicker. Proper calibration of the VCOM voltage also maintains a substantially zero volt average across the pixel which prevents the pixel material from becoming damaged, such as causing an image to be burned into the display screen.
The VCOM voltage is supplied using an appropriate VCOM application circuit. During the assembly of each conventional VCOM display panel, the VCOM voltage level can be calibrated by electronically adjusting parameters associated with the VCOM application circuit. In some processes, a technician views a test image on the LCD and manually adjusts the VCOM application circuit through empirical trial and error until the image does not have flicker. In other processes, the VCOM voltage calibration can be performed by monitoring the illumination of the VCOM display panel and inputting the detected illumination levels into a software algorithm to determine the proper VCOM voltage level. The determined proper VCOM voltage level is then set by the VCOM application circuit. The proper VCOM voltage level is then stored in non-volatile storage or set by a resistor string on the display controller board for the life of the VCOM display.
Conventional VCOM application circuits use a Class AB amplifier to generate the proper VCOM voltage level that is provided to the VCOM display panel. FIG. 1 illustrates an exemplary conventional VCOM application circuit 10. A digital-to-analog converter (DAC) 2 receives as input a digital code representative of the proper VCOM voltage level. The DAC 2 outputs a converted analog signal to a first input of an amplifier 4. The amplifier 4 is a Class AB operational amplifier. A second input of the amplifier 4 is a feedback signal. The amplifier 4 is supplied with an analog power supply voltage AVDD. An output of the amplifier 4 is the VCOM voltage level that is supplied to a backplane, or VCOM plane, of a LCD panel 20. The VCOM plane can be conceptually modeled as a distributed RC. In some applications, the VCOM voltage level is substantially constant. An alternative configuration of the VCOM application circuit 10′, as shown in FIG. 1B, can also be implemented to provide a constant VCOM voltage level. The VCOM application circuit 10′ includes a local feedback from the output of the Class AB amplifier 4′ to the second input of the Class AB amplifier 4′. The Class AB amplifier 4′ can be the same or different than the Class AB amplifier 4 in FIG. 1A. In other applications, the VCOM voltage level can be adjusted using the VCOM application circuit 10 (FIG. 1A) by providing a feedback signal from the VCOM plane 20 to the second input of the Class AB amplifier 4.
The VCOM plane distributes the VCOM voltage to each pixel within the LCD panel. In conventional applications, LCD panels include rows of pixels, each pixel in a row connected to a common gate line. As each row of pixels is refreshed, hundreds, if not thousands, of pixels may be simultaneously drawing current. Inclusion of the amplifier within VCOM application circuit accommodates the simultaneous drawing of current by a large number of pixels.
The output stage of a typical Class AB amplifier includes two complimentary transistors configured for sourcing and sinking current. The transistors in a Class AB amplifier operate in the linear mode. The power efficiency of the output stage is at best 50%.